1. Field of the Invention
This invention relates to the field of integrated device fabrication, and more particularly to the manufacture of integrated devices for use in microfluidics applications, such biological applications; in the latter case such devices are often known as biochips. Biochips require the fabrication of micro-channels for the processing of biological fluids, and the present invention relates a method of fabricating such channels.
2. Description of the Prior Art
The prior art is generally divided into two types of device: Passive and Active. Both types include microchannels for the transport of biological fluids. In passive devices all the control circuitry for fluid flow is on external circuitry. Active devices include control circuitry incorporated directly into the biochip.
The following granted U.S.A. Patents show the Prior Art concerning the fabrication of micro-channel biochips for the processing of biological fluids: U.S. Pat. No. 6,186,660, xe2x80x9cMicrofluidic systems incorporating varied channel dimensionsxe2x80x9d; U.S. Pat. No. 6,180,536, xe2x80x9cSuspended moving channels and channel actuators for . . . xe2x80x9d; U.S. Pat. No. 6,174,675, xe2x80x9cElectrical current for controlling fluid parameters in . . . xe2x80x9d; U.S. Pat. No. 6,172,353, xe2x80x9cSystem and method for measuring low power signalsxe2x80x9d; U.S. Pat. No. 6,171,865, xe2x80x9cSimultaneous analyte determination and reference balancing . . . ; U.S. Pat. No. 6,171,850, xe2x80x9cIntegrated devices and systems for performing temperature . . . xe2x80x9d; U.S. Pat. No. 6,171,067, xe2x80x9cMicropumpxe2x80x9d; U.S. Pat. No. 6,170,981, xe2x80x9cIn situ micromachined mixer for microfluidic analytical . . . xe2x80x9d; U.S. Pat. No. 6,167,910, xe2x80x9cMulti-layer microfluidic devicesxe2x80x9d; U.S. Pat. No. 6,159,739, xe2x80x9cDevice and method for 3-dimensional alignment of particles . . . xe2x80x9d; U.S. Pat. No. 6,156,181, xe2x80x9cControlled fluid transport microfabricated polymeric substratesxe2x80x9d; U.S. Pat. No. 6,154,226, xe2x80x9cParallel print arrayxe2x80x9d; U.S. patent No. substratesxe2x80x9d; U.S. Pat. No. 6,154,226, xe2x80x9cParallel print arrayxe2x80x9d; U.S. Pat. No. 6,153,073, xe2x80x9cMicrofluidic devices incorporating improved channel . . . xe2x80x9d; U.S. Pat. No. 6,150,180, xe2x80x9cHigh throughput screening assay systems in microscale . . . xe2x80x9d; U.S. Pat. No. 6,150,119, xe2x80x9cOptimized high-throughput analytical system xe2x80x9d; U.S. Pat. No. 6,149,870, xe2x80x9cApparatus for in situ concentration and/or dilution of . . . xe2x80x9d; U.S. Pat. No. 6,149,787, xe2x80x9cExternal material accession systems and methodsxe2x80x9d; U.S. Pat. No. 6,148,508, xe2x80x9cMethod of making a capillary for electrokinetic transport of . . . xe2x80x9d; U.S. Pat. No. 6,146,103, xe2x80x9cMicromachined magnetohydrodynamic actuators and sensors xe2x80x9d; U.S. Pat. No. 6,143,248, xe2x80x9cCapillary microvalvexe2x80x9d; U.S. Pat. No. 6,143,152, xe2x80x9cMicrofabricated capillary array electrophoresis device and . . . xe2x80x9d; U.S. Pat. No. 6,137,501, xe2x80x9cAddressing circuitry for microfluidic printing apparatusxe2x80x9d; U.S. Pat. No. 6,136,272, xe2x80x9cDevice for rapidly joining and splitting fluid layersxe2x80x9d; U.S. Pat. No. 6,136,212, xe2x80x9cPolymer-based micromachining for microfluidic devicesxe2x80x9d; U.S. Pat. No. 6,132,685, xe2x80x9cHigh throughput microfluidic systems and methodsxe2x80x9d; U.S. Pat. No. 6,131,410, xe2x80x9cVacuum fusion bonding of glass platesxe2x80x9d; U.S. Pat. No. 6,130,098, xe2x80x9cMoving microdropletsxe2x80x9d; U.S. Pat. No. 6,129,854, xe2x80x9cLow temperature material bonding techniquexe2x80x9d; U.S. Pat. No. 6,129,826, xe2x80x9cMethods and systems for enhanced fluid transportxe2x80x9d; U.S. Pat. No. 6,126,765, xe2x80x9cMethod of producing microchannel/microcavity structuresxe2x80x9d; U.S. Pat. No. 6,126,140, xe2x80x9cMonolithic bi-directional microvalve with enclosed drive . . . xe2x80x9d; U.S. Pat. No. 6,123,798, xe2x80x9cMethods of fabricating polymeric structures incorporating . . . xe2x80x9d; U.S. Pat. No. 6,120,666, xe2x80x9cMicrofabricated device and method for multiplexed . . . xe2x80x9d; U.S. Pat. No. 6,118,126, xe2x80x9cMethod for enhancing fluorescencexe2x80x9d; U.S. Pat. No. 6,107,044, xe2x80x9cApparatus and methods for sequencing nucleic acids in . . . xe2x80x9d; U.S. Pat. No. 6,106,685, xe2x80x9cElectrode combinations for pumping fluidsxe2x80x9d; U.S. Pat. No. 6,103,199, xe2x80x9cCapillary electroflow apparatus and methodxe2x80x9d; U.S. Pat. No. 6,100,541, xe2x80x9cMicrofluidic devices and systems incorporating integrated . . . xe2x80x9d; U.S. Pat. No. 6,096,656, xe2x80x9cFormation of microchannels from low-temperature . . . xe2x80x9d; U.S. Pat. No. 6,091,502, xe2x80x9cDevice and method for performing spectral measurements in . . . xe2x80x9d; U.S. Pat. No. 6,090,251, xe2x80x9cMicrofabricated structures for facilitating fluid introduction . . . xe2x80x9d; U.S. Pat. No. 6,086,825, xe2x80x9cMicrofabricated structures for facilitating fluid introduction . . . xe2x80x9d; U.S. Pat. No. 6,086,740. xe2x80x9cMultiplexed microfluidic devices and systemsxe2x80x9d; U.S. Pat. No. 6,082,140, xe2x80x9cFusion bonding and alignment fixture xe2x80x9d; U.S. Pat. No. 6,080,295, xe2x80x9cElectropipettor and compensation means for electrophoretic . . . xe2x80x9d; U.S. Pat. No. 6,078,340, xe2x80x9cUsing silver salts and reducing reagents in microfluidic printingxe2x80x9d; U.S. Pat. No. 6,074,827, xe2x80x9cMicrofluidic method for nucleic acid purification and processingxe2x80x9d; U.S. Pat. No. 6,074,725, xe2x80x9cFabrication of microfluidic circuits by printing techniquesxe2x80x9d; U.S. Pat. No. 6,073,482, xe2x80x9cFluid flow modulexe2x80x9d; U.S. Pat. No. 6,071,478, xe2x80x9cAnalytical system and methodxe2x80x9d; U.S. Pat. No. 6,068,752, xe2x80x9cMicrofluidic devices incorporating improved channel . . . xe2x80x9d; U.S. Pat. No. 6,063,589, xe2x80x9cDevices and methods for using centripetal acceleration to . . . xe2x80x9d; U.S. Pat. No. 6,062,261, xe2x80x9cMicrofluIdic circuit designs for performing electrokinetic . . . xe2x80x9d; U.S. Pat. No. 6,057,149, xe2x80x9cMicroscale devices and reactions in microscale devicesxe2x80x9d; U.S. Pat. No. 6,056,269, xe2x80x9cMicrominiature valve having silicon diaphragmxe2x80x9d; U.S. Pat. No. 6,054,277, xe2x80x9cIntegrated microchip genetic testing systemxe2x80x9d; U.S. Pat. No. 6,048,734, xe2x80x9cThermal microvalves in a fluid flow methodxe2x80x9d; U.S. Pat. No. 6,048,498, xe2x80x9cMicrofluidic devices and systemsxe2x80x9d; U.S. Pat. No. 6,046,056, xe2x80x9cHigh throughput screening assay systems in microscale . . . xe2x80x9d; U.S. Pat. No. 6,043,080, xe2x80x9cIntegrated nucleic acid diagnostic device xe2x80x9d; U.S. Pat. No. 6,042,710, xe2x80x9cMethods and compositions for performing molecular separationsxe2x80x9d; U.S. Pat. No. 6,042,709, xe2x80x9cMicrofluidic sampling system and methodsxe2x80x9d; U.S. Pat. No. 6,012,902, xe2x80x9cMicropumpxe2x80x9d; U.S. Pat. No. 6,011,252, xe2x80x9cMethod and apparatus for detecting low light levelsxe2x80x9d; U.S. Pat. No. 6,007,775, xe2x80x9cMultiple analyte diffusion based chemical sensorxe2x80x9d; U.S. Pat. No. 6,004,515, xe2x80x9cMethods and apparatus for in situ concentration and/or . . . xe2x80x9d; U.S. Pat. No. 6,001,231, xe2x80x9cMethods and systems for monitoring and controlling fluid . . . xe2x80x9d; U.S. Pat. No. 5,992,820, xe2x80x9cFlow control in microfluidics devices by controlled bubble . . . xe2x80x9d; U.S. Pat. No. 5,989,402, xe2x80x9cController/detector interfaces for microfluidic systemsxe2x80x9d; U.S. Pat. No. 5,980,719, xe2x80x9cElectrohydrodynamic receptorxe2x80x9d; U.S. Pat. No. 5,972,710, xe2x80x9cMicrofabricated diffusion-based chemical sensorxe2x80x9d; U.S. Pat. No. 5,972,187, xe2x80x9cElectropipettor and compensation means for electrophoretic biasxe2x80x9d; U.S. Pat. No. 5,965,410, xe2x80x9cElectrical current for controlling fluid parameters in . . . xe2x80x9d; U.S. Pat. No. 5,965,001, xe2x80x9cVariable control of electroosmotic and/or electrophoretic . . . xe2x80x9d; U.S. Pat. No. 5,964,995, xe2x80x9cMethods and systems for enhanced fluid transportxe2x80x9d; U.S. Pat. No. 5,958,694, xe2x80x9cApparatus and methods for sequencing nucleic acids in . . . xe2x80x9d; U.S. Pat. No. 5,958,203, xe2x80x9cElectropipettor and compensation means for electrophoretic biasxe2x80x9d; U.S. Pat. No. 5,957,579, xe2x80x9cMicrofluidic systems incorporating varied channel dimensionsxe2x80x9d; U.S. Pat. No. 5,955,028, xe2x80x9cAnalytical system and methodxe2x80x9d; U.S. Pat. No. 5,948,684, xe2x80x9cSimultaneous analyte determination and reference balancing . . . xe2x80x9d; U.S. Pat. No. 5,948,227, xe2x80x9cMethods and systems for performing electrophoretic . . . xe2x80x9d; U.S. Pat. No. 5,942,443, xe2x80x9cHigh throughput screening assay systems in microscalexe2x80x9d; U.S. Pat. No. 5,932,315, xe2x80x9cMicrofluidic structure assembly with mating microfeaturesxe2x80x9d; U.S. Pat. No. 5,932,100, xe2x80x9cMicrofabricated differential extraction device and method . . . xe2x80x9d; U.S. Pat. No. 5,922,604, xe2x80x9cThin reaction chambers for containing and handling liquid . . . xe2x80x9d; U.S. Pat. No. 5,922,210, xe2x80x9cTangential flow planar microfabricated fluid filter and method . . . xe2x80x9d; U.S. Pat. No. 5,885,470, xe2x80x9cControlled fluid transport in microfabricated polymeric . . . xe2x80x9d; U.S. Pat. No. 5,882,465, xe2x80x9cMethod of manufacturing microfluidic devicesxe2x80x9d; U.S. Pat. No. 5,880,071, xe2x80x9cElectropipettor and compensation means for electrophoretic biasxe2x80x9d; U.S. Pat. No. 5,876,675, xe2x80x9cMicrofluidic devices and systemsxe2x80x9d; U.S. Pat. No. 5,869,004, xe2x80x9cMethods and apparatus for in situ concentration and/or . . . xe2x80x9d; U.S. Pat. No. 5,863,502, xe2x80x9cParallel reaction cassette and associated devicesxe2x80x9d; U.S. Pat. No. 5,856,174, xe2x80x9cIntegrated nucleic acid diagnostic devicexe2x80x9d; U.S. Pat. No. 5,855,801, xe2x80x9cIC-processed microneedlesxe2x80x9d; U.S. Pat. No. 5,852,495, xe2x80x9cFourier detection of species migrating in a microchannelxe2x80x9d; U.S. Pat. No. 5,849,208, xe2x80x9cMaking apparatus for conducting biochemical analysesxe2x80x9d; U.S. Pat. No. 5,842,787, xe2x80x9cMicrofluidic systems incorporating varied channel dimensionsxe2x80x9d; U.S. Pat. No. 5,800,690, xe2x80x9cVariable control of electroosmotic and/or electrophoretic . . . xe2x80x9d; U.S. Pat. No. 5,779,868, xe2x80x9cElectropipettor and compensation means for electrophoretic biasxe2x80x9d; U.S. Pat. No. 5,755,942, xe2x80x9cPartitioned microelectronic device arrayxe2x80x9d; U.S. Pat. No. 5,716,852, xe2x80x9cMicrofabricated diffusion-based chemical sensorxe2x80x9d; U.S. Pat. No. 5,705,018, xe2x80x9cMicromachined peristaltic pumpxe2x80x9d; U.S. Pat. No. 5,699,157, xe2x80x9cFourier detection of species migrating in a microchannelxe2x80x9d; U.S. Pat. No. 5,591,139, xe2x80x9cxe2x80x9cIC-processed microneedlesxe2x80x9d; and U.S. Pat. No. 5,376,252, xe2x80x9cMicrofluidic structure and process for its manufacturexe2x80x9d.
The following published paper describes a polydimethylsiloxane (PDMS) biochip capable of capacitance detection of biological entities (mouse cells): L. L. Sohn, O. A. Saleh, G. R. Facer, A. J. Beavis, R. S. Allan, and D. A. Notterman, xe2x80x98Capacitance cytometry: Measuring biological cells one by onexe2x80x99, Proceedings of the National Academy of Siences (USA), Vol. 97, No. 20, Sep. 26, 2000, pp.10687-10690
The above US patents indicate that passive micro-channel biochip devices are largely fabricated from the combination of various polymer substrates, such as: acrylonitrile-butadiene-styrene copolymer, polycarbonate, polydimethylsiloxane (PDMS), polyethylene, polymethylmethacrylate (PMMA), polymethylpentene, polypropylene, polystyrene, polysulfone, polytetrafluoroethylene (PTFE), polyurethane, polyvinylchloride (PVC), polyvinylidine fluoride (PVF), or other polymer. In this case, lithography or mechanical stamping is used to define a network of micro-channels in one of these substrates, prior to the assembly and the thermally assisted bonding of this first substrate to another substrate. The result is a simple passive micro-channel biochip device which can be patterned with conductive layers for connection to an external processor that is used to initiate fluid movement by electrophoresis or electroosmosis, and for analysis and data generation. FIG. 1 shows an example of such a passive micro-channel biochip device obtained from the fusion of such polymeric substrates described in U.S. Pat. No. 6,167,910.
The prior art US patents also show that passive micro-channel biochip devices can be fabricated from the combination of various micro-machined silica or quartz substrates. Again, assembly and fusion bonding is required. The result is a simple passive biochip device which can be patterned with conductive layers for connection to an external processor. FIG. 2 shows an example of such passive micro-channel biochip device obtained from the fusion of such silica substrates as described in U.S. Pat. No. 6,131,410.
These prior art patents also show that passive micro-channel biochip devices can be fabricated from a passive micro-machined silicon substrate. In that case, the silicon substrate is used as a passive structural material. Again, assembly and fusion bonding of at least two sub-assemblies is required. The result is a simple passive biochip that has to be connected to an external processor. FIG. 3 shows an example of such a passive micro-channel biochip devices obtained from a passive micro-machined silicon substrate in accordance with the teachings of U.S. Pat. No. 5,705,018.
The prior patents also indicate that an active micro-reservoir biochip device can be fabricated from an active micro-machined silicon substrate. In this case, the control electronics integrated in the silicon substrate is used as an active on-chip fluid processor and communication device. The result is a sophisticated biochip which can perform, in pre-defined reservoirs, various fluidic operations, analysis and (remote) data communication functions without the need for an external fluid processor controlling fluid movement, analysis and data generation. FIG. 4 shows an example of an active micro-reservoir biochip devices obtained from an active micro-machined silicon substrate described in U.S. Pat. No. 6,117,643.
The published paper discloses that capacitance detection of biological entities can be performed on passive polydimethylsiloxane (PDMS) biochips using gold coated capacitor electrodes at a relatively low frequency of 1 kHz with and external detector. FIG. 5 shows an example of such passive polydimethylsiloxane (PDMS) biochips with gold electrodes.
The present invention relates to an improved fabrication technique of active micro-channel biochip devices from an active micro-machined silicon substrate that results in a sophisticated biochip device which can perform fluid movement and biological entities detection into micro-channels.
According to the present invention there is provided a method of fabricating a microstructure for microfluidics applications, comprising forming a layer of etchable material on a suitable substrate; forming a mechanically stable support layer over said etchable material; applying a mask over said support layer to expose at least one opening; performing an anistropic etch through the or each said opening to create a bore extending through said support layer into said layer of etchable material; performing an isotropic etch through the or each said bore to form a microchannel in said etchable material extending under said support layer; and forming a further layer of depositable material over said support layer until portions of said depositable layer overhanging the or each said opening meet and thereby close the microchannel formed under the or each said opening.
The invention involves the formation of a structure comprising a stack of layers. It will be appreciated by one skilled in the art that the critical layers do not necessarily have to be deposited directly on top of each other. It is possible that in certain applications intervenving layers may be present, and indeed in the preferred embodiment such layers, for example, a sacrificial TiN layer, are present under the support layer.
The invention offers a simple approach for the fabrication of active micro-channel biochip devices from an active micro-machined silicon substrate directly over a Complementary Metal Oxide Semiconductor device, CMOS device, or a high-voltage CMOS device.
CMOS devices are capable of very small detection levels, an important prerequisite in order to perform electronic capacitance detection (identification) of biological entities with low signal levels. CMOS devices can perform the required data processing and (remote) communication fonctions. High-voltage CMOS devices with adequate operation voltages and operation currents are capable of performing the required micro-fluidics in the micro-channels and allowing the integration of a complete Laboratory-On-A-Chip concept.
The invention discloses a technique for incorporating in existing CMOS and high-voltage CMOS processes the micro-machining steps which allow the development of the active micro-channels with attached electrodes used to provoke fluid movement and/or to identify biological entities. The micro-channels are closed using without the use of a second substrate and without the use of thermal bonding. In fact, all of the described micro-machining steps should preferably be carried out at a temperature not exceeding 450xc2x0 C. in order to prevent the degradation of the underlying CMOS and high-voltage CMOS devices and, prevent any mechanical problems such as plastic deformation, peeling, cracking, de-lamination and other such high temperature related problems with the thin layers used in the micro-machining of the bio-chip.
The materials combination used in the described micro-machining sequence are not typical of Micro-Electro-Mechanical-Systems (MEMS) which typically use Low Pressure Chemical Vapour Deposited polysilicon, LPCVD polysilicon, and Plasma Enhanced Chemical Vapour Deposited silica, PECVD SiO2, combinations. The use of LPCVD polysilicon is generally not suitable because of its required deposition temperature of more than 550xc2x0 C.
The invention preferably employs as an innovative sacrificial material Collimated Reactive Physical Vapour Deposition of Titanium Nitride, CRPVD TiN. In this process the TiN is deposited with the assistance of a collimator, which directs the atoms onto the supporting surface. This sacrificial CRPVD TiN material is used because of its excellent mechanical properties, and its excellent selectivity to Isotropic Wet Etching solutions used to define the micro-channels in thick layers of Plasma Enhanced Chemical Vapour Deposited, PECVD, SiO2.
Typically, the capacitor electrodes are either LPCVD polysilicon (deposited before the micro-machining steps) or Physical Vapour Deposited aluminum alloy, PVD Al-alloy.